A number of different imaging technologies have been employed for recording images from digital data onto photosensitive media. Conventional printing apparatus adapted for this purpose have used Cathode Ray Tubes (CRTs), scanned laser beams, Liquid Crystal Displays (LCDs), and Digital Micromirror Devices (DMDs). Each of these technologies, at its current level of maturity, is known to have inherent limitations.
In a CRT-based printer, digital image data is used to modulate a cathode ray tube which provides exposure energy by scanning an electron beam of variable intensity along its phosphorescent screen. CRT systems such as that described in U.S. Pat. Nos. 4,754,334 and 5,303,056 are relatively large and slow. Other problems such as non-uniform illumination, geometric distortion, complexity, cost, and size are among the more serious constraints of CRT-based printing approaches.
Alternately, light exposure energy can be applied from a laser-based printer, as is disclosed in U.S. Pat. No. 4,728,965. In a laser-based printer, digital image data is used to modulate laser on-time duration or intensity as the beam is scanned onto the imaging plane by a rotating polygon, or scanner. The scanner builds an image one pixel at a time. Limitations in systems using laser scanners, such as that described in U.S. Pat. No. 5,296,958, are due primarily to limitations in scanner speed. For increased speed, the raster scan system must be relatively complex in its construction. There are practical limitations in how laser scanner systems can be scaled in order to gain speed. The need for special media presents another limitation for usefulness of these systems. Other limiting problems with laser scan solutions include geometric distortion, complexity, and cost.
Spatial light modulators present another alternative imaging solution. A spatial light modulator can be considered essentially as a one-dimensional (linear) or two-dimensional (area) array of light-valve elements, each element corresponding to an image pixel. Spatial light modulators have been developed and used for relatively low resolution applications such as digital projection systems and for image display in portable devices such as helmet-mounted displays. One type of two-dimensional or area spatial light modulator being widely used in image projection applications is the DMD from Texas Instruments, Dallas, Tex. However, the resolution currently available with digital micromirror devices, as shown in U.S. Pat. Nos. 5,061,049 and 5,461,411, is not sufficient for high-quality printing needs such as for photographic prints or for motion picture film, for example. In practice, the requirements for projection and display differ significantly from the requirements for high resolution printing to a photosensitive medium, as would be required, for example, for motion pictures, for medical imaging, for micrographics, or for commercial photography. There appears no clear technology path for providing increased DMD resolution. Moreover, DMDs are expensive and are not easily scaled to higher resolution.
Another type of spatial light modulator being used in projection and printing applications is the two-dimensional LCD. Examples of printing apparatus using LCD spatial light modulators are disclosed in commonly-assigned U.S. Pat. Nos. 5,652,661 and 6,215,547. There are two basic types of LCD spatial light modulators currently in use, transmissive and reflective. Both types of LCD components modulate an incident optical beam for imaging by modulating the polarization state of the light. Polarization considerations are, therefore, important in the overall design of support optics for LCD spatial light modulators. Illumination optics must be adapted to provide highly uniform levels of intensity, with light at the proper polarization for modulation. This adds cost and complexity to the design of a printing apparatus using LCD components.
A recent development which offers considerable promise for both large-scale displays and small, handheld microdisplays is Organic Light-Emitting Diode (OLED), using organic electroluminscence technology. The light emitting material itself may be characterized as being of the small-molecule kind or of the polymer kind. In some of the current literature, small-molecule organic electroluminescent devices are labeled as “OLED” devices and distinguished from polymer organic electroluminescent devices, labeled “PLED” devices. For the purposes of this application, however, all of these classes of organic electroluminescent devices, including both small-molecule and polymer varieties, are simply referred to as “OLED” and can be considered for use as an electroluminescent image source. Individual OLEDs have been integrated on the same substrate to form high resolution area arrays. An organic light emitting diode array can be fabricated as a two dimensional monochromatic array of pixels or as a tri-color side by side pixel array, or even as a tri-color stacked pixel array. The array sizes, pixel pitch and aspect ratios can be manufactured in a variety of resolutions and densities. Each pixel site in a tri-color side by side or stacked array comprises three light emitting diodes of different colors. Each light emitting diode in a tri-pixel site, and therefore the entire array, can be individually controlled to create a full-color visual image. Alternately, three monochromatic arrays, each of different color, can be combined to provide the three component colors needed for a full-color visual image.
In contrast to LCD and other types of light modulators, which require an external illumination source with supporting optics and, in many cases, supporting polarization components, OLED arrays emit modulated light directly. Thus, when using OLED arrays, supporting illumination and polarization optics are not needed. This makes OLED devices particularly advantageous for use in display applications.
As is well known to those skilled in the imaging arts, imaging requirements for display differ significantly from imaging requirements for printing. Displays are optimized to provide maximum luminous flux to a screen, with secondary emphasis placed on characteristics important in printing, such as contrast and resolution. Optical systems for display applications are designed for the response of the human eye, which, when viewing a display, is relatively insensitive to image artifacts and aberrations and to image non-uniformity, since the displayed image is continually refreshed and is viewed from a distance. However, when viewing printed output from a high-resolution printing system, the human eye is not nearly as “forgiving” to artifacts and aberrations and to non-uniformity, since irregularities in optical response are more readily visible and objectionable on printed output. For this reason, there can be considerable complexity in optical systems for providing a uniform exposure energy for printing. Even more significant are differences in resolution requirements. Adapted for the human eye, display systems are optimized for viewing at typical resolutions such as 72 dpi or less, for example. Printing apparatus, on the other hand, must achieve much higher resolution, particularly apparatus designed for micrographics applications, which can be expected to provide 8,000 dpi for some systems. This places significantly different requirements on supporting optics for printing systems.
Primarily due to the emphasis on OLED use for display applications, there is, as yet, only limited interest in using OLED devices with printing systems. For example, U.S. Pat. No. 6,195,115 teaches the use of a linear array of OLED elements in the exposure head of color printers. In U.S. Pat. Nos. 6,072,517 and 6,137,523, a xerographic printhead incorporates an OLED area array but in a linear printing scheme, where light from multiple OLED elements in the same column are integrated to provide grayscale exposure for a single pixel. In both of U.S. Pat. Nos. 6,195,115 and 6,137,523 disclosures, OLEDs are used to print data one line at a time. Such systems not only have limited productivity, but also require precision media transport and synchronized data electronics, both of which could be complex and costly.
The OLED-based printing systems disclosed in U.S. Pat. No. 6,195,115 and U.S. Pat. Nos. 6,072,517 and 6,137,523 are directed to linear printing, in which a single line at a time is exposed as a photosensitive medium is moved through the print area. However, this type of printing fails to take advantage of the capability of the OLED emissive array to emit light for a full image frame at a time. With conventional linear printing, throughput is constrained and media transport systems become an important part of the imaging chain and are, therefore, more complex and costly. Moreover, simultaneous printing of multiple images is not possible with conventional linear techniques. Conventional linear printing techniques are also constrained with respect to image size and resolution.
U.S. Pat. No. 6,243,125 and European Patent Application EP 1 008 453 disclose a portable optical printer that employs two linear rows of luminous dots for providing exposure; the printhead is moved relative to the media in a direction perpendicular to the fluorescent luminous array to print a two-dimensional image. Just like the aforementioned U.S. Pat. Nos. 6,195,115; 6,072,517; and 6,137,523 disclosures, U.S. Pat. No. 6,243,125 and European Patent Application EP 1 008 453 are constrained in productivity and are disadvantaged in requiring complex and expensive media transport systems. Furthermore, the printhead disclosed in U.S. Pat. No. 6,195,115 provides only 1× imaging.
Thus, it can be seen that while CRT, LCD, laser, and other technologies have provided solutions for printing images from digital data onto photosensitive media, there is room for improvement. The use of two-dimensional emissive arrays, such as OLEDs, in a printing configuration in which a full image frame is printed at one time offers a possible printing solution with potential advantages over other printer technologies. However, there are, as yet, no guidelines for how to integrate OLED image-forming devices into an apparatus for printing on photosensitive media, where the printing apparatus provides, with sufficient throughput, a high-quality, high-resolution print output in a range of possible image formats.
Thus, it can be seen that there is a need for a printing apparatus that takes advantage of the full-frame imaging capability of the OLED for improved throughput, increased image format possibilities, improved image resolution, and improved capabilities for increased image size.